The present invention relates to holographic recording cells which make it possible to record holograms in a reversible manner in an electro-optical sensitive medium.
Recording of a hologram in a material such as lithium niobate (LiNbO.sub.3) is already known. The principal disadvantage of this material, and of similar materials, used to date, is their low sensitivity compared, for example, to those of photographic emulsions used for the same purpose.
It is also known that images can be recorded in electro-optical and photo-conducting materials such as bismuth-silicon oxide Bi.sub.12 SiO.sub.20 and bismuth-germanium oxide B.sub.12 GeO.sub.20 by applying a longitudinal electric field perpendicularly to a sheet of such material. This field is generally obtained by using two transparent electrodes fixed on the faces of the sheet. The image is projected on the sheet and recorded thereon in the form of local variations in the refractive index. This image can then be examined under a light which does not produce the photo-conductor effect and by using two crossed polarizers in order to display the index variations. Unfortunately such a device has a low resolution because of the migration of photo-electric charges in the thickness of the sheet.
If, however, a holographic recording cell is used which includes a sheet of one of these two materials subjected to a transverse electric field, parallel to a preferred crystal axis of this sheet, then holograms can be recorded in volume with a high resolution and a high sensitivity, both comparable to the characteristics of photographic emulsions.
However, as the crystal is only partially illuminated, illuminated and shaded zones occur and thus a variable local conductivity is created. This causes a partition in the voltage applied to the crystal, leading thereby to a very low field in the middle of the illuminated zone. Thus, if the reference beam does not illuminate the crystal uniformly, the application of the electric field only has a very slight effect on the efficiency of the hologram. Furthermore, the illumination of the crystal, as a function of the position along one axis of the crystal, is a Gaussian signal. The conductivity therefore follows the same variation as the electric field. The index variation in the crystal may thus be likened to a lens which produces a distortion on the conjugate beam which is clearly visible when the conjugate beam, which is the beam obtained after transmission through the electro-optical medium and deflection from a mirror, is compared with the incident beam.
In order to reduce these disadvantages, according to the invention the crystal is illuminated with an incoherent light, illuminating only part of the crystal with the laser beam and giving at one and the same time the advantages of a concentrated beam and of a wide angle beam. That is, it gives a high power density and a good beam geometry as well as an effective application of the electric field without creating a lens effect.